1. Field of the Invention
This invention relates generally to a grid structure, and is more particularly directed to an improved grid structure usable in a color cathode ray tube for determining the landing positions of one or more electron beams on a phosphor screen of the color cathode ray tube.
2. Description of the Prior Art
In existing color cathode ray tubes, such as, tubes of the type identified commercially by the trademark Trinitron, a grid structure, frequently called an aperture grill, is provided within the tube envelope in facing relation to the phosphor screen formed on the inner surface of the tube's face plate. Such grid structure is provided to ensure that electron beams emitted from a gun located in the neck portion of the tube envelope will be made to land on corresponding color phosphor stripes on the screen as the latter is scanned.
In accordance with the prior art, the grid structure includes a frame comprised of a pair of opposing support bars which extend laterally adjacent the top and bottom of the screen, and brace members disposed at opposite sides of the frame and connecting the support bars. The grid structure further comprises a large number of fine, wire-like grid elements extending parallel to each other in the vertical direction and being stretched between the support bars to which the grid elements are welded or otherwise secured. The grid elements are heated and expanded longitudinally during operation of the color cathode ray tube by reason of the impingement of the grid elements of the electron beam or beams scanning the phosphor screen. Therefore, in the inoperative or cool condition of the grid structure, the grid elements need to be longitudinally tensioned or stressed between the support bars by amounts sufficient to ensure that the grid elements will remain taut, and thus accurately positioned relative to each other, even when heated and expanded. In order to provide the desired longitudinal stressing or tensioning of the grid elements, the latter are welded or secured to the support bars at a time when the latter are displaced toward each other by loads applied thereto sufficient to effect resilient flexing of the brace members. After welding of the grid elements to the support bars, the loads urging the support bars toward each other are withdrawn so that the brace members tend to resiliently return to their original positions and cause the desired longitudinal stressing of the grid elements between the support bars.
In the above-described existing grid structure, the frame is made of a suitable steel, such as, carbon steel or the like, so as to be capable of withstanding the loads applied thereto during the welding of the grid elements to the frame, and also the loading of the frame resulting from the longitudinal stressing of the grid elements when the frame is completed. In order to provide the requisite strength and also sufficient resilient flexing of the frame under the influence of the loads applied thereto while securing the grid elements thereto, the support bars are conveniently provided with L-shaped cross-sections, while the brace members are given C-shaped configurations and formed of bar stock of rectangular cross-section. Such grid structure, in which the support bars and C-shaped brace members are formed separately and when welded together to form an integral structure and in which the tensioning of the grid elements results mainly from the resiliency of the C-shaped brace members, is relatively complicated in structure, expensive and heavy, and present difficulties in maintaining the desired manufacturing tolerances and accuracy.
When it has been attempted to avoid the above problems in the case of the grid structures of small cathode ray tubes by employing a grid structure frame of one-piece construction which is pressed or stamped to have a generally rectangular ring-like configuration, either the strength of the frame is insufficient to withstand the loading resulting from the tensioned grid elements or, if the cross-sectional shape and thickness of the pressed or stamped frame have been selected to provide the frame with a strength sufficient for that purpose, the resulting frame does not have adequate resiliency for imparting the necessary longitudinal stresses or tension to the grid elements, particularly to the grid elements located adjacent the opposite side portions of the grid structure.